Battery cell and redox flow battery

ABSTRACT

The battery cell for a flow battery includes a cell frame including a frame including a through-window and a manifold serving as an electrolyte flow path, and a bipolar plate blocking the through-window; a positive electrode disposed on one surface side of the bipolar plate; and a negative electrode disposed on another surface side of the bipolar plate. In this battery cell, in the frame, a thickness of a portion in which the manifold is formed is defined as Ft; in the bipolar plate, a thickness of a portion blocking the through-window is defined as Bt; in the positive electrode, a thickness of a portion facing the bipolar plate is defined as Pt; in the negative electrode, a thickness of a portion facing the bipolar plate is defined as Nt; and these thicknesses satisfy Ft≥4 mm, Bt≥Ft−3.0 mm, Pt≤1.5 mm, and Nt≤1.5 mm.

TECHNICAL FIELD

The present invention relates to a battery cell for a flow battery, and a redox flow battery using the battery cell.

BACKGROUND ART

One of high-capacity storage batteries for storing electricity of new energy from solar photovoltaic power generation and wind power generation is a flow battery, typically a redox flow battery (RF battery). The RF battery is charged or discharged using the oxidation-reduction potential difference between ions contained in a positive electrolyte and ions contained in a negative electrolyte (refer to, for example, Patent Literature 1). As illustrated in FIG. 9 of the principle of the operation of a RF battery α, the RF battery α includes a battery cell 100 divided into a positive electrode cell 102 and a negative electrode cell 103 by a proton-permeable membrane 101. The positive electrode cell 102 contains a positive electrode 104, and is connected via ducts 108 and 110 to a positive electrolyte tank 106, which stores a positive electrolyte. The duct 108 is equipped with a pump 112. These members 106, 108, 110, and 112 constitute a positive circulation line 100P, which circulates the positive electrolyte. Similarly, the negative electrode cell 103 contains a negative electrode 105, and is connected via ducts 109 and 111 to a negative electrolyte tank 107, which stores a negative electrolyte. The duct 109 is equipped with a pump 113. These members 107, 109, 111, and 113 constitute a negative circulation line 100N, which circulates the negative electrolyte. The electrolytes stored in the tanks 106 and 107 are circulated by the pumps 112 and 113 through the cells 102 and 103 during charge and discharge. When charge or discharge is not performed, the pumps 112 and 113 are stopped and the electrolytes are not circulated.

In general, the battery cell 100 is formed within a structure referred to as a battery cell stack 200 in FIG. 10. The battery cell stack 200 is constituted by sandwiching, from both sides, a multilayer structure referred to as a substack 200 s with two end plates 210 and 220, and fastening the substack 200 s with a fastening mechanism 230 (FIG. 10 illustrates a configuration using plural substacks 200 s). As illustrated in the upper part of FIG. 10, the substack 200 s has a configuration in which cell units constituted by a cell frame 120, a positive electrode 104, a membrane 101, a negative electrode 105, and a cell frame 120 are stacked, and the stack body is sandwiched between supply/drainage plates 190 and 190 (refer to the lower part of FIG. 10). Such a cell frame 120 in the cell unit includes a frame 122 including a through-window and a bipolar plate 121 blocking the through-window. The positive electrode 104 is disposed on and in contact with one surface side of the bipolar plate 121. The negative electrode 105 is disposed on and in contact with the other surface side of the bipolar plate 121. In this configuration, a single battery cell 100 is formed between bipolar plates 121 of adjacent cell frames 120.

In the substack 200 s, supply and drainage of electrolytes through the supply/drainage plates 190 and 190 to and from the battery cells 100 are performed with liquid supply manifolds 123 and 124 and liquid drainage manifolds 125 and 126, which are formed in the frames 122. The positive electrolyte is supplied from the liquid supply manifold 123 through an inlet slit formed in one surface side (exposed side in the drawing) of the frame 122 to the positive electrode 104, and drained through an outlet slit formed in an upper portion of the frame 122 to the liquid drainage manifold 125. Similarly, the negative electrolyte is supplied from the liquid supply manifold 124 through an inlet slit (represented by dotted lines) formed in the other surface side (hidden side in the drawing) of the frame 122 to the negative electrode 105, and drained through an outlet slit (represented by dotted lines) formed in an upper portion of the frame 122 to the liquid drainage manifold 126. Ring-shaped sealing members 127 such as O-rings or flat gaskets are disposed between the cell frames 120 to suppress leakage of electrolytes from the substack 200 s.

Input and output of electric power between the external apparatus and the battery cells 100 in the substacks 200 s are performed with a current-collecting structure using current collector plates formed of a conductive material. A pair of current collector plates is disposed for each of the substacks 200 s; and the current collector plates are individually electrically connected to, among plural cell frames 120 stacked, the bipolar plates 121 of cell frames 120 that are disposed at both ends in the stack direction.

CITATION LIST Patent Literature

PTL 1: Japanese Unexamined Patent Application Publication No. 2014-139905

SUMMARY OF INVENTION Technical Problem

In recent years, an increase in the demand for redox flow batteries as units for storing electricity of new energy has been expected, and there has been a demand for a battery cell having high battery performance. For example, it is expected that achievement of a decrease in the internal resistance of a battery cell leads to further enhancement of the battery performance of the battery cell.

The present invention has been made under the above-described circumstances. An object of the present invention is to provide a battery cell having high battery performance.

Solution to Problem

A battery cell according to an embodiment of the present invention is a battery cell for a flow battery, the battery cell including a cell frame including a frame including a through-window and a manifold serving as an electrolyte flow path, and a bipolar plate blocking the through-window; a positive electrode disposed on one surface side of the bipolar plate; and a negative electrode disposed on another surface side of the bipolar plate. In this battery cell, in the frame, a thickness of a portion in which the manifold is formed is defined as Ft; in the bipolar plate, a thickness of a portion blocking the through-window is defined as Bt; in the positive electrode, a thickness of a portion facing the bipolar plate is defined as Pt; in the negative electrode, a thickness of a portion facing the bipolar plate is defined as Nt; and these thicknesses satisfy the following formulae: Ft≥4 mm, Bt≥Ft−3.0 mm, Pt≤1.5 mm, and Nt≤1.5 mm.

A redox flow battery according to an embodiment of the present invention includes a cell stack in which plural battery cells described above are stacked; a positive circulation line configured to circulate a positive electrolyte through the cell stack; and a negative circulation line configured to circulate a negative electrolyte through the cell stack.

Advantageous Effects of Invention

The battery cell and the redox flow battery have low internal resistances and have high battery performance.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view illustrating the configuration of a cell frame of a battery cell in Embodiment 1.

FIG. 2 is a sectional view taken along II-II in FIG. 1.

FIG. 3 is a schematic view illustrating the configurations of a frame and a bipolar plate of a cell frame of a battery cell in Embodiment 2.

FIG. 4 is a schematic view illustrating the configuration of a cell frame that is an assembly composed of the frame and the bipolar plate in FIG. 3.

FIG. 5 is a sectional view taken along V-V in FIG. 4.

FIG. 6 is a schematic view illustrating the configurations of a frame and a bipolar plate of a cell frame of a battery cell in Embodiment 3.

FIG. 7 is a schematic view illustrating the configuration of a cell frame that is an assembly composed of the frame and the bipolar plate in FIG. 6.

FIG. 8 is a sectional view taken along VIII-VIII in FIG. 7.

FIG. 9 illustrates the principle of the operation of a redox flow battery.

FIG. 10 is a schematic view illustrating the configuration of a battery cell stack.

DESCRIPTION OF EMBODIMENTS Description of Embodiments of the Present Invention

Contents of embodiments according to the present invention will be first listed and described.

Through studies on a battery cell having high battery performance, the inventors of the present invention focused on the electric resistance of the electrodes. This is because the inventors have found that, the larger the thicknesses of electrodes disposed in a battery cell, the higher the internal resistance of the battery cell. Accordingly, the inventors examined combinations of the thicknesses of elements (including electrodes) of a battery cell, and have completed battery cells listed below.

<1> A battery cell according to an embodiment is a battery cell for a flow battery, the battery cell including a cell frame including a frame including a through-window and a manifold serving as an electrolyte flow path, and a bipolar plate blocking the through-window; a positive electrode disposed on one surface side of the bipolar plate; and a negative electrode disposed on another surface side of the bipolar plate. In this battery cell, in the frame, a thickness of a portion in which the manifold is formed is defined as Ft; in the bipolar plate, a thickness of a portion blocking the through-window is defined as Bt; in the positive electrode, a thickness of a portion facing the bipolar plate is defined as Pt; in the negative electrode, a thickness of a portion facing the bipolar plate is defined as Nt; and these thicknesses satisfy the following formulae: Ft≥4 mm, Bt≥Ft−3.0 mm, Pt≤1.5 mm, and Nt≤1.5 mm.

The battery cell has high battery performance. This is because the electrode thicknesses Pt and Nt are small thicknesses of 1.5 mm or less, which results in suppression of an increase in the internal resistance of the battery cell due to large electrode thicknesses. Conventionally, in general, the bipolar plate has a thickness of about 0.6 mm to about 1 mm, and electrodes disposed on both sides of the thin bipolar plate have a thickness of about 2 mm to about 3 mm. These thicknesses of the bipolar plate and electrodes have been determined in order to decrease the electric resistance of the bipolar plate and to suppress the pressure loss of electrolytes supplied to the electrodes.

<2> The battery cell according to the embodiment may have a configuration in which a flow channel serving as an electrolyte channel is formed on each of the one surface side and the other surface side of the bipolar plate.

Formation of flow channels in the surfaces of the bipolar plate enables electrolytes, supplied through the manifolds of the frame to the bipolar plate, to be rapidly distributed to the whole surfaces of the bipolar plate. As a result, the electrolytes are sufficiently supplied to the whole surfaces of the electrodes disposed on the surfaces of the bipolar plate, which enhances the battery performance of the battery cell. In particular, the thinner the electrodes, the higher the flow resistance against electrolytes in the planar direction of the bipolar plate and the less the electrolytes are distributed to the whole surfaces of the bipolar plate. Accordingly, the thinner the electrodes, the more preferable it is to form flow channels in the bipolar plate.

<3> The battery cell according to the embodiment may have a configuration in which the frame includes, as an inner peripheral recess portion, a peripheral portion surrounding a whole periphery around the through-window and having a smaller thickness than another portion of the frame, and the bipolar plate includes an outer peripheral engagement portion extending along a whole outer periphery of the bipolar plate, having a predetermined width, and engaging with the inner peripheral recess portion.

With this configuration, by simply fitting the bipolar plate into the inner peripheral recess portion of the frame, the bipolar plate can be disposed over the through-window of the frame, and the bipolar plate can also be aligned with respect to the frame. This can increase the productivity of the battery cell.

<4> The battery cell according to the embodiment may have a configuration in which the outer peripheral engagement portion is formed so as to have a smaller thickness than another portion of the bipolar plate.

In the bipolar plate, the outer peripheral engagement portion engaging with the inner peripheral recess portion is formed as a thin portion that is thinner than the other portion of the bipolar plate, so that the bipolar plate fit in the frame can be held with stability.

<5> A redox flow battery according to an embodiment includes a cell stack in which plural battery cells according to the above-described embodiment are stacked; a positive circulation line configured to circulate a positive electrolyte through the cell stack; and a negative circulation line configured to circulate a negative electrolyte through the cell stack.

The redox flow battery has high battery performance. This is because the battery cells in the redox flow battery have higher battery performance than conventional ones.

DETAILS OF EMBODIMENTS OF THE PRESENT INVENTION

Hereinafter, redox flow batteries (RF batteries) according to embodiments will be described. However, the scope of the present invention is not limited to the configurations of the embodiments, but is indicated by Claims. The scope of the present invention is intended to embrace all the modifications within the meaning and range of equivalency of the Claims.

Embodiment 1

As with the conventional RF battery a described with reference to FIG. 9, a RF battery according to this embodiment includes a battery cell 100, a positive circulation line 100P, and a negative circulation line 100N. The battery cell 100 of this embodiment is used in the form of the battery cell stack 200 in FIG. 10. As described above, the battery cell stack 200 has a configuration in which plural cell units each including a membrane 101, electrodes 104 and 105, and a pair of cell frames 120 and 120 are stacked. A main difference of the RF battery of this embodiment from the conventional one lies in the thicknesses of the cell frames and the electrodes of such a cell unit. Hereinafter, a cell frame 1 and electrodes 104 and 105 according to this embodiment will be described with reference to FIGS. 1 and 2. Incidentally, in the cell frame 1, the same elements as conventional ones are denoted by the same reference signs as in FIG. 10.

<<Cell Frame>>

As illustrated in FIG. 1, the cell frame 1 includes a frame 12 and a bipolar plate 11. The frame 12 includes a through-window 22 w extending through the frame 12 in the thickness direction. The bipolar plate 11 is disposed so as to fill the through-window 22 w. The outer periphery of the bipolar plate 11 is embedded within the inner peripheral portion around the through-window 22 w of the frame 12.

[Frame]

As illustrated in FIG. 1, the frame 12 is a member that supports the bipolar plate 11 described later. As in the conventional configuration, the frame 12 includes liquid supply manifolds 123 and 124, liquid drainage manifolds 125 and 126, inlet slits 123 s and 124 s, and outlet slits 125 s and 126 s. The inlet slit 123 s and the outlet slit 125 s, which are represented by solid lines, are disposed on the exposed side of the drawing. The inlet slit 124 s and the outlet slit 126 s, which are represented by dotted lines, are disposed on the hidden side of the drawing. The slits 123 s to 126 s respectively extend from the manifolds 123 to 126 toward the center line of the frame 12 and are connected to the through-window 22 w (the inlet slit 124 s and the outlet slit 126 s are partially not shown). The manifolds 123 to 126 and the slits 123 s to 126 s are surrounded by a sealing member 127 such as an O-ring to prevent electrolytes from leaking beyond the sealing member 127 to the outside. Such O-rings are compressed when plural cell frames 1 are stacked and fastened, and provide sealing functions. The sealing member 127 may be a double sealing member. Sealing members (not shown) may be disposed so as to surround the manifolds.

As illustrated in the partial sectional view of FIG. 2, the frame 12 of this example is formed by bonding together two frame-shaped plate members, which provides a sectional shape having symmetry with respect to the left and the right. The frame-shaped plate members have, on their through-window sides (lower sides in the drawing), thin portions. The two frame-shaped plate members are bonded together to form, between the thin portions of the frame-shaped plate members, a space for housing the outer peripheral portion of the bipolar plate 11.

The material for the frame 12 preferably has a highly insulating property, more preferably further has acid resistance. Examples of the material for the frame 12 include vinyl chloride, chlorinated polyethylene, and chlorinated paraffin.

[Bipolar Plate]

As illustrated in the sectional view of FIG. 2, the bipolar plate 11 has one surface side in contact with the positive electrode 104, and the other surface side in contact with the negative electrode 105. The bipolar plate 11 of this example is a plate member substantially having a constant thickness. Among stacked bipolar plates 11, the bipolar plates 11 that are disposed at ends are in contact with and electrically connected to current collector plates.

As illustrated in FIG. 1, the one surface side and the other surface side of the bipolar plate 11 of this embodiment have interdigitated flow channels 11 g and 11 g for distributing electrolytes supplied via the inlet slits 123 s and 124 s to the whole surfaces of the bipolar plate 11. These two flow channels 11 g and 11 g are disposed so as to have an interdigitated comb-teeth configuration. The drawing shows the flow channels 11 g and 11 g only on the right side of the bipolar plate 11; actually, another pair of flow channels is formed on the left side of the bipolar plate 11. The flow channels on the left side are disposed so as to have line symmetry together with the flow channels 11 g and 11 g shown in the drawing as if these flow channels 11 g and 11 g were shifted beyond the center line of the bipolar plate 11. The interdigitated flow channels 11 g and 11 g enable rapid distribution of the electrolytes, supplied via the inlet slits 123 s (124 s) to the bipolar plate 11, over the whole surfaces of the bipolar plate 11. Thus, in FIG. 2, the electrolytes can be distributed over the whole surfaces of the positive electrode 104 and the negative electrode 105 disposed on the one surface side and the other surface side of the bipolar plate 11. Accordingly, even when the electrodes 104 and 105 are formed with reduced thicknesses, the battery performance of the battery cell is not degraded.

Incidentally, the shape of the flow channels 11 g is not limited to the comb-teeth shape illustrated, and may be any shape as long as electrolytes can be distributed over the whole surfaces of the bipolar plate 11. For example, the flow channels may have a dendritic shape.

As illustrated in FIG. 2, the outer peripheral portion of the bipolar plate 11 is sandwiched between the two frame-shaped plate members constituting the frame 12. This sandwiching fixes the bipolar plate 11 so as to be joined to the frame 12. The outer peripheral portion of the bipolar plate 11 has grooves, and O-rings 210 are disposed in the grooves. This sealing structure suppresses flow of electrolytes between the one surface side and the other surface side of the bipolar plate 11.

The material for the bipolar plate 11 preferably has high electrical conductivity, more preferably further has acid resistance and flexibility. For example, the material is a conductive material containing a carbonaceous material. Specifically, the material may be a conductive plastic composed of graphite and a chlorinated organic compound, or may be such a conductive plastic in which the graphite is partially substituted with at least one of carbon black and diamond-like carbon. Examples of the chlorinated organic compound include vinyl chloride, chlorinated polyethylene, and chlorinated paraffin. Such a material is used to constitute the bipolar plate 11, to thereby provide the bipolar plate 11 that has low electric resistance, and has high acid resistance and high flexibility.

<<Electrodes>>

As illustrated in the partial sectional view of FIG. 2, the positive electrode 104 and the negative electrode 105 are respectively disposed on the one surface side (the right side of the drawing) and the other surface side (the left side of the drawing) of the bipolar plate 11. The electrodes 104 and 105 are both porous bodies having deformation properties, and are compressed between the stacked cell frames 1. The drawing is illustrated as if there were gaps between the electrodes 104 and 105 and their adjacent members; actually, no such gaps are formed due to the deformation properties of the electrodes 104 and 105.

The material for the electrodes 104 and 105 preferably has high electrical conductivity, more preferably further has acid resistance. For example, woven fabric or non-woven fabric formed of carbonaceous fibers may be used to constitute the electrodes 104 and 105. Alternatively, for example, carbon paper sheets may be used as the electrodes 104 and 105.

<<Thicknesses of Frame, Bipolar Plate, and Electrodes>>

The cell frame 1 and the electrodes 104 and 105 are formed such that, in the frame 12, the thickness of a portion in which a manifold (represented by dotted lines in the drawing) is formed is defined as Ft; in the bipolar plate 11, the thickness of a portion blocking the through-window is defined as Bt; in the positive electrode 104, the thickness of a portion facing the bipolar plate 11 is defined as Pt; in the negative electrode 105, the thickness of a portion facing the bipolar plate 11 is defined as Nt; and these thicknesses satisfy the following formulae: Ft≥4 mm, Bt≥Ft−3.0 mm, Pt≤1.5 mm, and Nt≤1.5 mm.

The frame 12 is formed with the thickness Ft of 4 mm or more. As illustrated in FIG. 1, the manifolds 123 to 126, the slits 123 s to 126 s, and the like are formed in the frame 12. Accordingly, the frame 12 needs to have a relatively large thickness to thereby maintain the strength of the frame 12. When Ft is excessively large, the part that does not contribute to charge or discharge has an excessively large thickness, which results in a decrease in the charge-discharge capacity per volume of the battery cell. For this reason, the upper limit value of Ft is set to 8 mm. Considering the balance between strength and charge-discharge capacity, Ft is preferably 4 mm or more and 6 mm or less.

The bipolar plate 11 is formed with the thickness Bt, which is equal to or more than Ft−3.0 mm. Since Ft≈Bt+Pt+Nt, the larger the thickness Bt, the smaller the thickness Pt of the positive electrode 104 and the thickness Nt of the negative electrode 105. Bt may be equal to or more than Ft−1.0 mm. When the thickness Bt is excessively large, the electrodes 104 and 105 become excessively thin. For this reason, the upper limit value of Bt is preferably Ft−0.5 mm.

The thickness Pt of the positive electrode 104 and the thickness Nt of the negative electrode 105 are set to 1.5 mm or less. By forming the electrodes 104 and 105 as thin electrodes, an increase in the internal resistance of the battery cell can be effectively suppressed. This effect becomes stronger as the thicknesses of the electrodes 104 and 105 are decreased. For this reason, Pt and Nt are preferably set to 1.0 mm or less, more preferably 0.60 mm or less, still more preferably 0.30 mm or less. On the other hand, the lower limit values of Pt and Nt are preferably set to 0.25 mm in view of handleability of the electrodes 104 and 105. Incidentally, Pt and Nt are not necessarily the same value.

<<Advantages>>

The configuration having been described so far can provide a battery cell having higher battery performance than conventional ones. This is because the electrodes 104 and 105 of the battery cell have small thicknesses, which enables suppression of an increase in the internal resistance of the battery cell due to the electrodes 104 and 105.

Embodiment 2

Embodiment 2 will be described with reference to FIGS. 3 to 5 in terms of an embodiment including a cell frame 2 having a configuration different from that in Embodiment 1. FIG. 3 is a schematic view of a frame 22 and a bipolar plate 21 that constitute the cell frame 2. FIG. 4 is a schematic view of the cell frame 2 in which the bipolar plate 21 is attached to the frame 22. FIG. 5 is a sectional view taken along V-V in FIG. 4. In these drawings, flow channels formed in the surfaces of the bipolar plate 21 are not shown.

The difference of the cell frame 2 from Embodiment 1 lies in employment of a structure in which a peripheral portion, around the through-window 22 w, of the frame 22 engages with the outer periphery of the bipolar plate 21, that is, employment of an inset structure in which the bipolar plate 21 is fit into the frame 22. Accordingly, the inset structure and the configuration associated therewith will be mainly described below. Obviously, even in the case of employing the inset structure, the thickness Ft of the frame 22, the thickness Bt of the bipolar plate 21, the thickness Pt of the positive electrode 104, and the thickness Nt of the negative electrode 105 are selected so as to satisfy the values having been described in the embodiment, to suppress an increase in the internal resistance of the battery cell.

[Inset Structure]

The inset structure is constituted in the following manner: the dimensions of the cell frame 2 are adjusted such that, in the cell frame 2 viewed from its front side, the outer peripheral portion (along the whole periphery) of the bipolar plate 21, which is disposed so as to block the through-window 22 w of the frame 22, overlaps the frame 22; and a portion of the frame 22, the portion being overlapped by the bipolar plate 21, is formed as a recessed portion. In this example, a peripheral portion of the frame 22, the peripheral portion extending along the whole periphery around the through-window 22 w, is thinner than the other portion of the frame 22; and this thin portion forms an inner peripheral recess portion 22 c into which the bipolar plate 21 is fit. The inner peripheral recess portion 22 c in this example is formed only on one surface side of the frame 22. In other words, the backside surface of the inner peripheral recess portion 22 c flatly extends to a portion outside the backside surface (refer to FIG. 5).

The bipolar plate 21 is fit into the inner peripheral recess portion 22 c, so that, as illustrated in FIG. 4, the inner peripheral recess portion 22 c engages with, in the thickness direction of the frame 22, the outer peripheral engagement portion (extending over the whole outer periphery and having a predetermined width) of the bipolar plate 21 (refer also to FIG. 5). As a result, the through-window 22 w of the frame 22 is blocked with the bipolar plate 21. As illustrated in FIG. 5, in this case of employing the inset structure, in order to prevent flow of electrolytes between one surface side and the other surface side of the bipolar plate 21, sealing needs to be provided between the frame 22 and the bipolar plate 21. In this example, a ring-shaped groove is formed in a portion of the bipolar plate 21, the portion overlapping the inner peripheral recess portion 22 c, and an O-ring 210 is disposed in the groove to thereby form a sealing structure. Such O-rings 210 are compressed when plural cell frames 2 are stacked and fastened, and provide sealing functions. Alternatively, such a sealing structure may be formed with a flat gasket, or by thermal fusion, or by coating the inner peripheral recess portion 22 c with an adhesive and bonding together the inner peripheral recess portion 22 c and the bipolar plate 21.

As illustrated in FIG. 4, in the case of employing the inset structure, by simply fitting the bipolar plate 21 into the inner peripheral recess portion 22 c of the frame 22, the bipolar plate 21 can be disposed over the through-window 22 w of the frame 22. In addition, by fitting the bipolar plate 21 into the inner peripheral recess portion 22 c, the bipolar plate 21 can be aligned with respect to the frame 22. This enables an increase in the productivity of the cell frame 2.

In the case of employing the inset structure, in the presence of tolerance during production, it is difficult to make the outer size of the inner peripheral recess portion 22 c be equal to the outer size of the bipolar plate 21; and if these outer sizes are made to be equal to each other, it becomes difficult to fit the bipolar plate 21 into the frame 22, which is problematic. Accordingly, the outer size of the inner peripheral recess portion 22 c is made slightly larger (by, for example, about 1 mm to about 1.5 mm) than the outer size of the bipolar plate 21, to facilitate fitting of the bipolar plate 21 into the frame 22. However, in this case, a leakage channel 9 of the electrolyte is formed in the cell frame 2 viewed from its front side, the leakage channel 9 being formed between the frame 22 and the bipolar plate 21, the leakage channel 9 extending from the inlet slit 123 s to the outlet slit 125 s. The leakage channel 9 is a gap between the members and has a very low flow resistance. Thus, the electrolyte introduced through the inlet slit 123 s to the bipolar plate 21 tends to flow into the leakage channel 9. The electrolyte flowing into the leakage channel 9 substantially does not come into contact with the positive electrode on the bipolar plate 21, and is drained through the outlet slit 125 s. Accordingly, the larger the amount of the electrolyte flowing in the leakage channel 9, the lower the charge-discharge efficiency of the battery cell. For this reason, the following dividing structure (not shown) that divides the leakage channel 9 is preferably provided.

[Dividing Structure]

As illustrated in FIG. 4, the leakage channel 9 is constituted by a first horizontal channel 9 d disposed in a lower portion of the cell frame 2 and connected to the inlet slit 123 s, a second horizontal channel 9 u disposed in an upper portion of the cell frame 2 and connected to the outlet slit 125 s, and two vertical channels 9 sr and 9 sl that connect together the two horizontal channels 9 d and 9 u. Dividing structures that divide the leakage channel 9 are broadly grouped into the following three configurations:

(1) a configuration in which a dividing member is stuffed into the leakage channel 9 to divide the leakage channel 9;

(2) a configuration in which the bipolar plate 21 has a portion protruding toward the frame 22 and the protruding portion divides the leakage channel 9; and

(3) a configuration in which the frame 22 has a portion protruding toward the bipolar plate 21 and the protruding portion divides the leakage channel 9.

Among these three configurations, in particular, (1) the dividing member will be described. The dividing member is preferably constituted by an elastic member that has deformation properties and can be pressed into the leakage channel 9. For example, a long rubber member may be used as the dividing member. In the leakage channel 9, the position where the dividing member is disposed is not particularly limited. For example, such dividing members may be fit into lower portions of the vertical channels 9 sr and 9 sl (closer to the first horizontal channel 9 d). In this case, the electrolyte introduced into the first horizontal channel 9 d rapidly spreads through the first horizontal channel 9 d, which results in uniform distribution in the width direction of the bipolar plate 21 (in the left-right direction in the drawing). Subsequently, the electrolyte flowing through the vertical channel 9 sr (9 sl) hits against the dividing member and flows in the center direction (toward the electrode) of the bipolar plate 21. As a result, the electrolyte comes into contact with the electrode disposed on the surface of the bipolar plate 21, to contribute to charge or discharge.

Embodiment 3

Embodiment 3 will be described with reference to FIGS. 6 to 8 in terms of a cell frame 3 having an inset structure slightly different from that of Embodiment 2. Main differences of the cell frame 3 of Embodiment 3 from Embodiment 2 are that a portion of a bipolar plate 31, the portion engaging with an inner peripheral recess portion 32 c of a frame 32 and having a predetermined width, is formed so as to be thinner than the other portion of the bipolar plate 31; and the leakage channel 9 is formed so as to partially meander. Hereinafter, the differences from Embodiment 2 will be mainly described. Obviously, also in this embodiment, the thickness Ft of the frame 32, the thickness Bt of the bipolar plate 31, the thickness Pt of the positive electrode 104, and the thickness Nt of the negative electrode 105 are selected so as to satisfy the values having been described in the embodiment, to suppress an increase in the internal resistance of the battery cell.

FIG. 6 is a schematic view of the frame 32 and the bipolar plate 31 in Embodiment 3. These frame 32 and bipolar plate 31 have configurations for making the leakage channel 9 meander. As a configuration for making the leakage channel 9 meander, the frame 32 of this embodiment includes first protrusions 32 x, which protrude toward the inner peripheral recess portion 32 c. The frame 32 further includes second protrusions 32 y, which protrude from the inner peripheral recess portion 32 c toward the through-window 22 w.

On the other hand, the bipolar plate 31 of this embodiment includes, as a configuration for making the leakage channel 9 meander, first recesses 31 x formed by cutting away portions that correspond to the first protrusions 32 x of the frame 32. In the back surface side of the bipolar plate 31, the outer peripheral engagement portion (portion outside a portion represented by a dotted line), which engages with the inner peripheral recess portion 32 c of the frame 32, is a thin portion 31 c formed so as to be thinner than the other portion of the bipolar plate 31. A surface (on the exposed side in the drawing) of the thin portion 31 c is flush with the other portion. Thus, a surface (on the hidden side in the drawing) of the thin portion 31 c is recessed, with respect to the other portion, toward the exposed side in the drawing. Portions of the thin portion 31 c that correspond to the second protrusions 32 y of the frame 32 include second recesses 31 y, which are formed so as to extend toward the center line of the bipolar plate 31.

As illustrated in FIGS. 7 and 8, when the bipolar plate 31 having the above-described configuration is fit into the frame 32, a leakage channel 9 is formed on one surface side (on the exposed side in FIG. 7, on the right side in FIG. 8) of the cell frame 3, and another leakage channel 9 is formed on the other surface side (on the hidden side in FIG. 7, on the left side in FIG. 8). These two leakage channels 9 are divided with dividing members (not shown).

In the configuration of Embodiment 3 having been described so far, as illustrated in FIG. 8, a portion of the bipolar plate 31 other than the thin portion 31 c is fit into the through-window of the frame 32, so that the bipolar plate 31 engages with the frame 32 with more stability than in Embodiment 2.

INDUSTRIAL APPLICABILITY

A battery cell according to the present invention is suitably applicable to formation of flow-type storage batteries such as RF batteries. A RF battery according to the present invention is usable as, in new-energy power generation such as solar photovoltaic power generation and wind power generation, storage batteries used for purposes such as stabilization of variations in output of generated power, storage of surplus power of generated power, and load leveling, and is also usable as high-capacity storage batteries that are placed adjacent to ordinary power plants and used for purposes of addressing voltage sag and power failure and achieving load leveling.

Reference Signs List   α redox flow battery (RF battery)   1, 2, and 3 cell frames    11, 21, and 31 bipolar plates      11g flow channel; 31c thin portion (outer peripheral engagement portion); 21o O-ring      31x first recess; 31y second recess    12, 22, and 32 frames      32c inner peripheral recess portion; 22w through-window      22x and 32x first protrusions; 32y second protrusion   9 leakage channel    9d first horizontal channel; 9u second horizontal channel    9sr and 9sl vertical channels   100 battery cell; 101 membrane; 102 positive electrode cell; 103 negative electrode cell    100P positive circulation line; 100N negative circulation line    104 positive electrode; 105 negative electrode; 106 positive electrolyte tank    107 negative electrolyte tank; 108, 109, 110, and 111 ducts    112 and 113 pumps    120 cell frame; 121 bipolar plate; 122 frame    123 and 124 liquid supply manifolds    125 and 126 liquid drainage manifolds    123s and 124s inlet slits; 125s and 126s outlet slits    127 sealing member    190 supply/drainage plate; 210 and 220 end plates 200 battery cell stack; 200s substack    230 fastening mechanism 

The invention claimed is:
 1. A battery cell for a flow battery, comprising: a cell frame including a frame including a through-window and a manifold serving as an electrolyte flow path, and a bipolar plate blocking the through-window; a positive electrode disposed on one surface side of the bipolar plate; and a negative electrode disposed on another surface side of the bipolar plate, wherein in the frame, a thickness of a portion in which the manifold is formed is defined as Ft; in the bipolar plate, a thickness of a portion blocking the through-window is defined as Bt; in the positive electrode, a thickness of a portion facing the bipolar plate is defined as Pt; in the negative electrode, a thickness of a portion facing the bipolar plate is defined as Nt; and these thicknesses satisfy Ft≥4 mm, Bt≥Ft−3.0 mm, Pt≤1.5 mm, and Nt≤1.5 mm.
 2. The battery cell according to claim 1, wherein a flow channel serving as an electrolyte channel is formed on each of the one surface side and the other surface side of the bipolar plate.
 3. The battery cell according to claim 1, wherein the frame includes, as an inner peripheral recess portion, a peripheral portion surrounding a whole periphery around the through-window and having a smaller thickness than another portion of the frame, and the bipolar plate includes an outer peripheral engagement portion extending along a whole outer periphery of the bipolar plate, having a predetermined width, and engaging with the inner peripheral recess portion.
 4. The battery cell according to claim 3, wherein the outer peripheral engagement portion is formed so as to have a smaller thickness than another portion of the bipolar plate.
 5. A redox flow battery comprising: a cell stack in which a plurality of the battery cells according to claim 1 are stacked; a positive circulation line configured to circulate a positive electrolyte through the cell stack; and a negative circulation line configured to circulate a negative electrolyte through the cell stack. 